Excitation-independent yellow-fluorescent nitrogen-doped carbon nanodots for biological imaging and paper-based sensing
Graphical abstract
Introduction
Carbon nanodots (CDs), fascinating fluorescent nanomaterials, have attracted increasing attention since they were serendipitously discovered during purification of single-walled carbon nanotubes in 2004 [1]. They are generally defined as carbon nanomaterials with sizes below 10 nm and considered to consist of an amorphous or crystalline core with predominant sp2 carbon and an oxidized carbon shell with oxygen-containing groups [2]. By virtue of their high aqueous solubility, tunable photoluminescence (PL), robust chemical inertness, easy functionalization, low environmental hazard, and excellent biocompatibility, CDs have gradually become superior alternatives to other fluorescent nanoparticles. These distinct benefits of CDs make them great promise for a variety of practical applications, such as bioimaging [3], [4], [5], [6], ink [7], sensing [8], [9], [10], [11], optoelectronics [12], and photocatalysis [13].
To date, there have been remarkable advances in the synthesis of CDs and their synthesis methods can be divided into two major types: top-down and bottom-up routes. Top-down approaches involve laser ablation or electrochemical oxidation of graphite, electrochemical treatment of multiwalled carbon nanotubes, and chemical oxidation of commercially activated carbon. Bottom-up methods include pyrolysis [14], [15], wet oxidation [16], hydrothermal synthesis [17], [18], and microwave-assisted synthesis [12], [19]. Out of these methods, hydrothermal synthesis has been proved to be an effective and convenient way for the synthesis of CDs with its simple experimental setup, easy control of the reaction, and low energy consumption.
Currently, most of CDs show intense emission only in the blue-light region, and the long-wavelength (i.e., yellow- to red-light) emissions are usually very weak. In the case of biology-relevant fields, CDs with blue luminescence might be less favorable as optical nanoprobes because of the commonly blue autofluorescence of biological matrix and photodamage of biological tissues by ultraviolet excitation light. Therefore, the preparation of CDs with long-wavelength emission is highly desirable. Recently, to get CDs with long-wavelength emission, heteroatom doping has been a subject of topical interest. Gong et al. [20] applied an effective approach to prepare phosphorous and nitrogen co-doped yellow luminescent CDs by acidic oxidation of pumpkin by H3PO4 at low temperature. Jiang et al. [21] reported bright-yellow-emissive nitrogen-doped CDs by solvothermal method of 1,2,4-triaminobenzene. Song et al. [22] described a synthetic approach for yellow fluorescent graphene quantum dots using o-phenylenediamine as carbon precursor. Cheng et al. [23] demonstrated a simple strategy for the generation of zinc ion-doped yellow luminescent CDs using zinc ions and citric acid as the precursor via a one-pot solvothermal method. Xu et al. [24] proposed an ingenious strategy for fabrication of yellow luminescent CDs by using glucose as carbon source and concentrated phosphoric acid as dehydrating agent. Chang et al. [25] obtained yellow fluorescence CDs from sucrose by acid carbonization with phosphoric acid. Nevertheless, most of these reported synthesis methods often require multi-step procedure, expensive raw materials, complicated and timeconsuming isolation or purification. Thus it is of urgent need to prepare CDs with strong long-wavelength emission by facile and economic methods.
Aluminium, the most abundant metal element in the earth’s crust, is a common metal frequently used in our daily life [26]. The wide use of aluminum inevitably results in a health risk such as Parkinson’s disease, Kidney damage, and Alzheimer’s disease. Thus the detection of Al3+ is very important from the view point of human health and food safety. Recently, some Al3+ probes have been reported. Chen et al. [27] presented the visual detection of Al3+ using unlabeled gold nanoparticles (AuNPs). Gui et al. [28] reported a fluorescence turn-on chemosensor 2-(4-(1,2,2-triphenylvinyl)phenoxy)acetic acid (TPE-COOH) for Al3+ with a detection limit of 21.6 nM. Liu et al. [29] developed a commercial 2-hydroxy-1-naphthaldehyde used as a highly sensitive and selective fluorescent sensor for Al3+ in EtOH-H2O solution. Saini et al. [30] demonstrated a “turn–on” approach for highly sensitive and selective detection of Al3+ using multifunctional salen type ligand as fluorescent probes. However, all these above-mentioned methods suffer from some drawbacks like the requirement of tedious synthetic methodology or the use of expensive starting materials. Therefore, the development of easy and economic method for the synthesis of Al3+ sensors remains challenging.
In the present work, bright yellow fluorescent nitrogen-doped CDs (y-CDs) have been prepared from 4-amino salicylic acid by one-step hydrothermal approach for the first time. The process is simple and efficient. As-prepared y-CDs exhibit strong, stable, excitation-independent, and yellow fluorescence. Coupled with excellent PL properties and low toxicity, obtained y-CDs have been successfully utilized to Al3+ detection in versatile environments, such as aqueous solution, living cells, and paper based sensor strips.
Section snippets
Materials
The 4-amino salicylic acid, NaH2PO4 and Na2HPO4 were purchased from Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). Rhodamin 6G was obtained from E. Merck, Darmstadt. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was obtained from Solarbio (Beijing, China). Other reagents were purchased from Beijing Chemical Corp. (Beijing, China). Distilled deionized (DDI) water was obtained from a Millipore Milli-Q-RO4 water purification system with a resistivity of 18.2 MΩ cm−1
The synthesis of y-CDs
The successful fabrication of y-CDs was carried out by using 4-amino salicylic acid as the starting material via one-step hydrothermal method (Fig. 1). Compared with traditional methods for fabrication of y-CDs using more than one raw material and requiring complicated and timeconsuming isolation or purification, our proposed method only utilizes 4-amino salicylic acid as the precursor which avoids the use of a large amount of strong acid and tedious post-treatment processes. Hydrothermal
Conclusions
We have described an easy route for fabricating y-CDs with the use of 4-amino salicylic acid as the precursor via one-pot hydrothermal synthesis. As-prepared y-CDs show not only high fluorescence quantum yield and extraordinary excitation-independent emission but low cytotoxicity and superior biocompatibility. Interestingly, obtained y-CDs have been developed as fluorescent nanosensors for rapid, precise, and quantitative sensing of Al3+ ions in aqueous solution. More importantly, confocal
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21575084, 21475080, 21306108, and 21305082), and Shanxi Scholarship Council of China (2014-018 and 2014-017), Innovative Talents in Higher School Support Plan(2014107), and Technology Foundation for Selected Overseas Chinese Scholar in Shanxi Province. We also acknowledge Dr. Juanjuan Wang from the Scientific Instrument Center at Shanxi University for her help with Zeiss LSM880 confocal laser-scanning microscope
Lihong Shi is now working as an associate professor in Shanxi University, Shanxi Province, China. She obtained her Ph.D. degree in Physical Chemistry from Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences in 2008. Her current research interests include nanomaterial and chemosensors.
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Lihong Shi is now working as an associate professor in Shanxi University, Shanxi Province, China. She obtained her Ph.D. degree in Physical Chemistry from Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences in 2008. Her current research interests include nanomaterial and chemosensors.
Lin Li obtained B.Sc. degree in Chemistry from Changzhi University in 2015. Now she is a postgraduate in Shanxi University and major in analytical chemistry.
Xiaofeng Li received BSc degree in chemistry from Shanxi Nomal University in 1999. Now he is a postgraduate in Shanxi University. His research lied in supermolecular chemsitry.
Guomei Zhang received her BSc degree from the Shanxi University of China (Taiyuan) in 1997, her MSc degree and PhD degree in analytical and inorganic chemistry in 2003 and 2006 from Shanxi University, respectively. She joined the Chemistry Faculty of Shanxi University in 2006, and master tutor at Shanxi University since 2008 and has been a professor in 2014. Her current research interests include the DNA assembly and photoluminescence analysis of nanomaterials.
Yan Zhang received is now working as an associate professor in Shanxi University, Shanxi Province, China. She obtained her PhD degree in analytical chemistry from Shanxi University in 2008. Her current research interests include metal nanomaterials.
Chuan Dong is a professor of Analytical Chemistry at Shanxi University. He obtained his Ph.D. degree at Shanxi University in 2002. Dong has published over 200 papers and 100 patents and his current research interests are biosensors, chemosensors, optical fiber sensor, fluorescence and room temperature phosphorescence, and environmental analysis and monitoring.
Shaomin Shuang is a professor of Analytical Chemistry at Shanxi University. She received her Ph.D. degree in South China University of Technology in 1998. She has published extensively in journals such as Nanoscale,Journal of Materials Chemistry B, Chemical Communications, Sensors and Actuators B, Analytical Chemistry, Talanta etc. Her research interests are in the areas of analytical chemistry, supermolecular chemsitry, and photochemsity.